The present invention relates to a method of producing a vacuum circuit breaker and, more particularly, to a method of producing a vacuum circuit breaker having electrodes or contacts made of an alloy containing as the main component copper or copper alloy and a sub-component which is a metal having a lower melting point and a higher vapour pressure than the main component, the sub-component having a solubility limit to the main component at the room temperature and being contained in excess of the solubility limit. Still more particularly, the invention is concerned with a method suitable for the production of a vacuum circuit breaker having contacts made of an alloy in which added to copper is lead, bismuth or an alloy of lead and bismuth, in excessive of its solubility limit to the copper at the room temperature.
In general, the following five characteristics are the essential requisites for the vacuum circuit breaker.
(1) High dielectric strength PA1 (2) Good interrupting ability PA1 (3) Superior non-welding characteristic of contacts PA1 (4) Low chopping current PA1 (5) Small gaseous content in the material of contacts
The improvements of these characteristics are generally achieved by improving the properties of the material of contacts. Conventionally, copper has been used as the major material for the contacts of vacuum circuit breakers. However, as a result of the above-mentioned improvement, an alloy containing copper as the main component to which added is iron or cobalt has been developed as a contact material having a superior dielectric strength. Also, as a contact material having superior non-welding property and a low chopping current, an alloy has been developed which contains copper as the main component and a metal having a lower melting point and a higher vapour pressure than the copper, as well as small solid solubility to copper at room temperature, e.g. lead, bismuth or an alloy of lead and bismuth.
The term "chopping" as appearing in the foregoing description of the properties is used to mean such a phenomenon that the current after the breaking is suddenly reduced to zero, before it naturally falls to zero. Thus, the current as observed just before the sudden reduction is referred to as the "chopping current". It is preferred to make the level of the chopping current as low as possible, for otherwise the insulation of the load may be broken. The chopping phenomenon is liable to occur when the arc generated at the time of breaking has a small stability. Therefore, in order to diminish the chopping current, it is suggested to add to the contact material a metal which is easily evaporated by the heat generated by the arc, so as to enhance the stability of the arc. The aforementioned alloy for copper containing lead, bismuth or an alloy of lead and bismuth can maintain a low chopping current, because lead, bismuth or an alloy of lead or bismuth is easily evaporated by the heat of arc, contributing to the stabilization of the arc. At the same time, the presence of lead, bismuth or their alloy in the surface of the contacts facilitate the separation of the contacts, even when they are happened to be welded to each other. Thus, this alloy has also a superior non-welding characteristics.
To explain in more detail about the function of the sub-component such as lead, bismuth or an alloy of lead and bismuth, the copper alloy having such a sub-component is usually produced by casting. In the course of this casting, since such a sub-component has almost no solubility limit to copper at room temperature, the sub-component is made to exist at the grain boundaries. This sub-component then exudes to the surface of the contact, due to its melting by the heat generated by the arc at the time of breaking. Although the presence of this sub-component contributes to the reduction of the chopping current and the improvement of the non-welding characteristic as stated before, too much exudation causes a contamination of the interior of the breaker, resulting in a reduced dielectric strength. In addition, the exudation is caused not only at the time of the breaking but also during the soldering of the contact to the contact holder, so as to deteriorate the solderability.
Therefore, in the alloy as the material of contacts containing a main component of copper or copper alloy and a sub-component of above explained metal, it is a new problem to be solved to reduce the exudation of the sub-component.